Electrically compensated constant speed drives (ECCSD's have been proposed for use in applications where conventional hydromechanical constant speed drives have been found inadequate. For example, Dishner, et al. U.S. Pat. No. 4,695,776, assigned to the assignee of the instant application and Borger U.S. Pat. No. 4,572,961 disclose ECCSD's including a differential having first and second input shafts coupled to a speed-compensation link and an output shaft at which the constant speed motive power is developed wherein a prime mover is coupled to the first input shaft. The speed-compensation link includes first and second permanent magnet machines interconnected by a power converter. In Dishner, et al., the power converter comprises a first AC/DC converter, a DC/DC converter and a second AC/DC converter. In Borger, the machines are interconnected by a power converter comprising either a DC link converter or a cycloconverter.
Dishner, et al. U.S. Pat. No. 4,692,671, assigned the assignee of the instant application discloses an electrically compensated constant speed drive of the "input differential" type wherein a prime mover is coupled to a first input shaft of a differential and a speed-compensation link is coupled between a second input shaft and an output shaft of the differential. The speed-compensation link includes first and second permanent magnet machines interconnected by an electrical power converter like that disclosed in the Dishner, et al. '776 patent described above.
In each of the foregoing ECCSD's, the power flow between the permanent magnet machines in the speed-compensation link is controlled so that compensating speed of a proper magnitude and direction is supplied to the second differential input shaft to keep the output shaft of the differential at a desired speed. This, in turn, causes a synchronous generator driven by the differential output shaft to develop constant frequency AC power.
Baker U.S. Pat. No. 4,694,187 discloses an electromechanical constant speed drive generating system wherein a first input shaft of a differential is coupled to an output shaft of a prime mover, an output shaft of the differential is coupled to a main generator and a speed-compensation link is coupled between armature windings of the main generator and a second input shaft of the differential. The speed-compensation link includes a dynamoelectric machine having a motive power shaft coupled to the second input of the differential and a power converter coupled between the main generator armature windings and electrical power windings of the dynamoelectric machine. Power flow between the electromechanical machine and the main generator armature windings is controlled so that the output shaft of the differential is maintained at a constant speed.
A power generating system which includes a constant speed drive having a single dynamoelectric machine and a power converter in the speed-compensation link thereof is disclosed in Cook, et al. U.S. patent application Ser. No. 15,903, filed Feb. 18, 1987, and entitled "Power Generating System". This speed-compensation link is also connected between armature
windings of a main generator driven by an output shaft of a differential and an input shaft of the differential.
Law U.S. Pat. No. 4,636,707 discloses a power generating system wherein a wind-driven turbine is coupled to a first input shaft of a differential having an output shaft coupled to a generator. A second input shaft of the differential is coupled to a synchronous generator which is in turn coupled to a thyristor-controlled variable load. The thyristors of the load are operated such that the energy of wind gusts on the turbine is absorbed partly by accelerating the turbine and partly by the thyristor-controlled load.
Constant speed drives have also been used to accelerate the prime mover from standstill up to self-sustaining speed. For example, Baker, et al. U.S. Pat. No. 4,697,090 discloses a starting system and method for the constant speed drive disclosed in the Dishner, et al. '776 patent. When operating in a starting mode, external power is provided to the permanent magnet machine coupled to the second differential input shaft so that this machine operates as a motor to accelerate the shaft. This, in turn, accelerates the output shaft of the differential so that the generator connected thereto is brought up to synchronous speed. Once synchronous speed is reached, external power is supplied to the generator so that it develops motive power. Thereafter, the permanent magnet machine coupled to the second differential shaft is operated so that it supplies the same magnitude of torque on the second differential input shaft as the magnitude of torque imposed on the differential output shaft by the generator. This balancing torque provided by the permanent magnet machine is developed by drawing power therefrom. The combined operation of the permanent magnet machine and the generator causes motive power to be returned through the differential to the prime mover to bring it up to self-sustaining speed.
The Cook, et al. system described above may also be utilized in a starting mode to start the prime mover. In this mode, power from an external power source is applied either directly to the electrical power windings of the main generator or to the windings of the main generator through the power converter in the speed-compensation link so that the motive power shaft of the main generator is accelerated toward synchronous speed. Rotation of the main generator motive power shaft in turn causes the motive power shaft of the dynamoelectric machine in the speed-compensation link to rotate in a direction opposite its direction of rotation when in the generating mode. Once synchronous speed of the main generator is reached, the main generator is connected directly to an external AC power source, if it is not already connected thereto, so that the generator operates as a synchronous motor. An electrical load is then applied to the dynamoelectric machine so that a balancing torque is developed. The generator and dynamoelectric machine together cause starting torque to be transferred through the differential to the prime mover to start same.
The Baker '187 patent likewise discloses that the system disclosed therein may be used in a starting mode. In this system, the main generator is operated as a motor and the power converter in the speed-compensation link is operated to transfer power from the electromagnetic machine coupled to the second differential input shaft. Motive power is thus developed which is transferred through the differential to the prime mover to start same.
Other types of generating systems have been developed which include prime mover start capability. Cronin U.S. Pat. No. 4,401,938 discloses the use of an induction machine driven by an engine and which operates in a generating mode to develop polyphase AC power whereby excitation for the induction machine is provided by a permanent magnet generator which is driven by a toroidal differential drive coupled to the output of the engine. The system is operable in a starting mode during which an engine starting circuit provides a programmed frequency and voltage to the induction machine to cause it to operate as a motor and thereby bring the engine up to operating speed.
Mehl U.S. Pat. No. 4,481,459, assigned to the assignee of the instant application discloses an engine starting and generating system wherein a rotor of a brushless, synchronous generator including a permanent magnet generator, an exciter and a main generator is coupled to the output of a prime mover by a torque converter. The system is operable in a starting mode by emptying the torque converter and by providing power to the permanent magnet generator to cause it to accelerate the rotor of the synchronous generator up to synchronous speed. Thereafter, power is applied to the generator to cause it to operate as a synchronous motor and thereby develop motive power. The torque converter is then filled so that the motive power is transmitted to the prime mover to bring it up to self-sustaining speed.